Role of 99mTc-(V)DMSA in detecting tumor cell proliferation.

Pentavalent technetium-99m dimercaptosuccinic acid ((99m)Tc-(V)DMSA) is a tumor-seeking agent which was introduced to evaluate, image, and manage many types of cancers. In this review, the beginning of, and the most recent applications of using this agent was appraised. The relation with tumor cell detection and proliferation was reported and several mechanisms of uptake of (99m)Tc-(V)DMSA in tumor cells are described.


Introduction
Recently attempts have been made to fi nd a more accurate and specifi c radioactive tracer to image the proliferative activity of various types of cancers, including breast cancer. 18 F-fl urodeoxy glucose ( 18 F-FDG) positron emission tomography (PET), 18 F-fl urothymidine ( 18 F-FLT), and deoxyuridine has shown unclear results (Rasey et al. 2002;Toyohara et al. 2002;Buck et al. 2002). Pentavalent technetium-99m dimercaptosuccinic acid ( 99m Tc-(V)DMSA) is a new tumor-seeking agent, which was introduced to evaluate, image, and manage medullary carcinoma of the thyroid (Ohta et al. 1984;Clarke et al. 1988). 99m Tc-(V)DMSA uptake showed its presence and effi cacy in detecting many other types of cancers; such as head and neck, in particular squamous cell carcinoma, and soft tissue tumors (Ohta et al. 1988;Watkinson et al. 1989), breast (Papantoniou et al. 2002;Kashyap et al. 1992), brain (Hirano et al. 1997), lung (Hirano et al. 1995;Atasever et al. 1997), bone (Lam et al. 1997), and in particular, for metastasis and high-grade tumors (Kiratli et al. 1998). However in patients with carcinomas of the liver and gastrointestinal tract, malignant melanoma and lymphoma, the role of 99m Tc-(V)DMSA was absent (Ohta et al. 1988). Physiological uptake has been evidenced in the kidneys, nasal mucosa, lacrimal glands and blood pool such as in the heart and vessels. Uptake of 99m Tc-(V)DMSA was also reported as normal distribution in the pituitary gland (Watkinson et al. 1990). It's uptake in the breasts has often been noted. Nakamoto et al. (1997) reported that female breasts should be included in the normal biodistribution of 99m Tc-(V)DMSA and should not be considered as pathological. 99m Tc-(V)DMSA has a very high affi nity for various tumors and increased uptake in tumors, but the mechanism of 99m Tc-(V)DMSA uptake in tumors is still not fully elucidated. A working hypothesis based on the metal-complex equilibrium has anticipated the pH sensitive character of 99m Tc-(V)DMSA. This character was suggested to be one of the factors that affected its accumulation in cancer cells (Horiuchi et al. 1998a).
Factually, tumours are well known to be more acidic than normal cells (Tannock and Rotin, 1989), in vivo 99m Tc-(V)DMSA uptake was correlated to the lowering of pH of tumors by glucose administration using Ehrlich ascites tumor cells (EATC)-bearing mice (Horiuchi et al. 1998b). The study found evidence of correlation between the 99m Tc-(V)DMSA uptake, which refl ects the tumor acidosis, and the 14 C-deoxyglucose ( 14 C-DG) uptake, which refl ects the tumor glycolysis and viability in EATC-bearing mice. Horiuchi et al. (1998b) reported that 99m Tc-(V)DMSA uptake by tumors was related to glucosemediated acidosis, and this may refl ect tumor cell viability. However, this was only reported by one study and in one type of cell lines (EATC) in vivo. This EATC also belongs to well-differentiated cells with very rapid growth rate and capacity of high aerobic glycolysis (Bustamante et al. 1981), and therefore it can not refl ect the accurate mechanism of 99m Tc-(V)DMSA uptake by tumors.
One study suggested that 99m Tc-(V)DMSA uptake by breast tumors is related to proliferative activity in breast, which is directly related to tumor grade (Papantoniou et al. 1999). In addition, Papantoniou Analytical Chemistry Insights 2007:2 81-83 et al. (2001) reported that 99m Tc-(V)DMSA uptake by breast tumors and several breast lesions is related to the mitotic activity and the cellular proliferation of breast tumors or lesions. Recently a strong correlation between 99m Tc-(V)DMSA uptake and cellular proliferation, as measured by Ki-67 expression (Ki-67, a monoclonal antibody that recognizes a labile epitope on a nuclear antigen expressed in cycling cells but not quiescent cells (Sawhney and Hall, 1992) was demonstrated (Papantoniou et al. 2004;Papantoniou et al. 2005). Papantoniou et al. (2004) showed that the proliferative activity is a major independent factor affecting 99m Tc-(V)DMSA uptake in breast cancer as determined by Ki-67 expression and suggested that 99m Tc-(V)DMSA uptake can be of clinical significance as an in vivo indicator of cell proliferation.
One in vitro study showed that 99m Tc-(V)DMSA uptake in cancer cell lines (human breast cancer; MCF-7, human glioblastoma multiform; G152, human fi brosarcoma; HT1080, lung adenocarcinoma; A549, human amelanomic melanoma; M3DAU, and grade III human glioblastoma; U-87-MG) is closely related to proliferation rate and focal adhesion kinase (FAK). Because proliferation rate and FAK are linked to cancer progression, Denoyer et al. (2005) assumed that in vivo 99m Tc-(V)DMSA uptake refl ects tumor aggressiveness and that 99m Tc-(V)DMSA uptake could provide clinicians with preoperative information not always obtainable by mammography.
In addition, its mechanism of uptake is thought to be due to the structural similarity between 99m Tc-(V)DMSA core (phosphate-like ion TcO 4 −3 ) and phosphate (PO 4 −3 ) anion as taken by some cancer cells (Yokoyama and Saji, 1980;Wulfrank et al. 1989). 99m Tc-(V)DMSA was originally designed as a metabolic mimic of phosphate, able to localize in cancer cells by supposed hydrolysis of the pentavalent DMSA complex within cancer cells to produce the phosphate-like ion TcO 4 −3 (Horiuchi et al. 1986;Yokoyama and Saji, 1980;Yokoyama et al. 1981;Yokoyama et al. 1985), then the hydrolysis of TcO 4 −3 and the subsequent metabolism or whatever mechanism starts in the cancer cells. The possible involvement of phosphate anion in 99m Tc-(V)DMSA uptake by tumors was confi rmed by the inhibition of the uptake of 99m Tc-(V)DMSA in the presence of phosphate ion (Lam et al. 1996).
Phosphate ion carries a negative charge, and so it is weakly accumulated in the cytosol by simple diffusion. However, it enters cells via three different types of NaPi co-transporters: type I NaPi co-transporters or NPT1 in humans, expressed in liver and kidney (Ghishan et al. 1993;Li and Xie, 1995), type II NaPi co-transporters (NPT2 in humans), expressed in brain (Hisano et al. 1997), osteoclasts (Gupta et al. 1996), lung (Magagnin et al. 1993), small intestine and proximal tubes (Hilfiker et al. 1998), and type III NaPi cotransporters, which are a family of cell surface receptors for the Gibbon ape leukaemia virus (GALV) (Kavanaugh and Kabat, 1996). These receptors are highly related to phosphate cotransporters (PiT1 in humans). This protein family is found in several organs and many tumor-forming tissues (Kavanaugh and Kabat, 1996;Kavanaugh et al.1994;Johann et al. 1992;Miller and Miller, 1994;Uckert et al. 1998).
One study by Palmedo et al. (1996;2000) demonstrated that 99m Tc-(V)DMSA is a marker of phosphate transport and 99m Tc-(V)DMSA enters a cancer cell line model specifi cally via type III NaPi co-transporters (PiT1) (Denoyer et al. 2004). This was recently demonstrated by Denoyer et al. (2004), by comparing 99m Tc-(V)DMSA and phosphate uptake kinetics in three cancer cell lines (human breast cancer; MCF-7, human glioblastoma multiform; G152, and human osteosarcoma; MG-63) and showed that 99m Tc-(V)DMSA uptake is specifi cally mediated by PiT1 in cancer cells. From this study, 99m Tc-(V)DMSA uptake can be assumed to be cell specifi c, because MCF-7 and G152 cancer cell lines exhibited the same 99m Tc-(V)DMSA uptake, whereas MG-63 cancer cell line showed the highest phosphate accumulation and the lowest 99m Tc-(V)DMSA uptake.

Conclusion
In this review, 99m Tc-(V)DMSA capability to detect cancer non invasively was reported. Several studies proposed the mechanism of 99m Tc-(V)DMSA uptake that might be involved by cancer cells. In addition, some studies reported its relation to cancer cell proliferation and this uptake might be used as a proliferation marker in cancer cells. However, until now, no strict mechanism has been fully clarifi ed.